Alignment of single-mode polymer waveguide (PWG) array and silicon waveguide (SIWG) array of providing adiabatic coupling

ABSTRACT

A method for fabricating, and a structure embodying, a single-mode polymer wave guide array aligned with a polymer waveguide array through adiabatic coupling. The present invention provides a structure having a combination of (i) a stub fabricated on a polymer and (ii) a groove fabricated on a silicon (Si) chip, with which an adiabatic coupling can be realized by aligning (a) a (single-mode) polymer waveguide (PWG) array fabricated on the polymer with (b) a silicon waveguide (SiWG) array fabricated on the silicon chip; wherein, the stub fabricated on the polymer is patterned according to a nano-imprint process, along with the PWG array, in a direction in which the PWG array is fabricated, and the groove fabricated on the silicon chip is fabricated along a direction in which the SiWG array is fabricated.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from JapanesePatent Application No. 2912-231097 filed Oct. 18, 2012, the entirecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to provision of a coupling allowingtransmission of light between a polymer waveguide (PWG) array and asilicon waveguide (SiWG) array. More specifically, the present inventionrelates to a technique of providing high-precision alignment of arraysin a single-mode polymer waveguide (PWG) so that an adiabatic couplingis provided.

DESCRIPTION OF THE RELATED ART

Both multimode and single-mode polymer waveguides (PWG) are widely usedeither in a rigid form on a printed board, or in a flexible form on apolymer base film.

The principle of optical waveguide (WG) is that a combination of coreand clad, which is a combination of two different type polymers having adifferent refractive index, is provided and the core is used as a lighttransmitting path.

On the other hand, silicon waveguides (SiWG) in which a lighttransmitting path is fabricated on a silicon (Si) chip are also widelyused.

Both in the polymer waveguide (PWG) and in the silicon waveguide (SiWG),multi-channel waveguides (WG) are fabricated in array in one directionin parallel with each other so that multi-channel light transmittingpaths are provided.

Attempts have been made to transmit light between the polymer waveguide(PWG) and the silicon waveguide (SiWG). However, a coupling which allowslight to be transmitted efficiently at the microscopic level requireshigh-precision positioning.

In the case of multimode waveguides (WG), when the waveguides arecoupled to each other or when the waveguide is coupled to a multimodeoptical fiber, a large cross section of core and almost the same size ofcore cross section and almost the same numerical apertures can providesuch coupling with an acceptable level of loss as long as a precisepositioning of the abutting cross sections is guaranteed between thecross sections coming into connection with each other.

Actually, what is called a “Butt Coupling” is used to provide the abovecoupling.

However, when a single-mode waveguide (WG) is coupled to a siliconwaveguide (SiWG), the cross section of core is very small and theconnected waveguides are very different from each other in the size ofcore cross section and in the numerical aperture. Thus, it is evendifficult to use the Butt Coupling.

In this case, adiabatic coupling may be used as an alternative method bywhich evanescent light in an optical axis direction along the array iscaptured and transmitted over a predetermined distance in the opticalaxis direction.

However, a technique is still unknown which, when a single-modewaveguide (WG) is coupled to a silicon waveguide (SiWG), provideshigh-precision alignment of arrays to provide the adiabatic coupling.

Concerning innovative methods for fabricating polymer waveguides (PWG)or positioning multi-channel polymer waveguides (PWG), a variety ofelemental techniques are known, such as those described in JapanesePatent Application Publications: No. 200075158A; No. 2004102220A; No.20085853A; No. 6109936A; No. 2006139147A; No. 2006139147A; No.2005208187A; No. 200889879A; No. 200931780A; No. 200931780A; No.2007212786A; No. 745811A; No. 11258455A.

However, a literature is still unknown which provides the adiabaticcoupling or refers to high-precision alignment such as self-alignmentfor providing the adiabatic coupling.

SUMMARY OF THE INVENTION

An object of the present invention is to align a (single-mode) polymerwaveguide (PWG) array fabricated on polymer and a silicon waveguide(SiWG) array fabricated on a silicon (Si) chip and thereby realize anadiabatic coupling.

The present invention provides a structure having a combination of (i) astub fabricated on a polymer and (ii) a groove fabricated on a silicon(Si) chip, with which an adiabatic coupling can be realized by aligning(a) a (single-mode) polymer waveguide (PWG) array fabricated on thepolymer with (b) a silicon waveguide (SiWG) array fabricated on thesilicon chip; wherein, the stub fabricated on the polymer is patternedaccording to a nano-imprint process, along with the PWG array, in adirection in which the PWG array is fabricated, and the groovefabricated on the silicon chip is fabricated along a direction in whichthe SiWG array is fabricated.

The present invention also discloses a method of fabricating on apolymer a (single-mode) polymer waveguide (PWG) array and a stub so thatthe (single-mode) polymer waveguide (PWG) array and the stub are alignedwith a silicon waveguide (SiWG) array fabricated on a silicon (Si) chipand a groove fabricated along a direction in which the SiWG isfabricated, whereby an adiabatic coupling is realized, the methodincludes the steps of: preparing a polymer base layer; placing on thepolymer base layer a cast having a groove corresponding to a core of the(single-mode) polymer waveguide (PWG) array and a groove correspondingto the stub; hardening the polymer base layer; and removing the casefrom the hardened polymer base layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view illustrating an area in which the inventiveadiabatic coupling is provided and the structure of a package containingthe area;

FIG. 2 is a top view for explaining overlapping of a silicon waveguide(SiWG) array and a polymer waveguide (PWG) array and for explainingevanescent wave involved in adiabatic coupling;

FIG. 3 is a top view for explaining the importance of alignment foroverlapping of a silicon waveguide (SiWG) array and a polymer waveguide(PWG) array;

FIG. 4 is an overall view illustrating a combination of a stubfabricated on polymer and a groove fabricated on a silicon chip, thecombination allowing alignment of a (single-mode) polymer waveguide(PWG) array fabricated on polymer and a silicon waveguide (SiWG) arrayfabricated on a silicon (Si) chip whereby the adiabatic coupling isrealized according to the present invention;

FIG. 5 is a view illustrating the inventive method of highly preciselyfabricating on polymer a (single-mode) polymer waveguide (PWG) array anda stub;

FIG. 6 is a view for explaining respective advantages of PWG patterningby photolithography and PWG patterning by nano-imprint when the polymerwaveguide (PWG) array and the stub are fabricated according to thepresent invention;

FIG. 7 is a view for explaining respective methods of PWG patterning byphotolithography and PWG patterning by nano-imprint when the polymerwaveguide (PWG) array and the stub are fabricated according to thepresent invention; and

FIG. 8 is a view for explaining a method of aligning the siliconwaveguide (SiWG) array and the polymer waveguide (PWG) array and thensecuring the arrays, and for explaining the state after the arrays havebeen secured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is an overall view illustrating an area in which the inventiveadiabatic coupling is provided and the structure of a package containingthe area.

Adiabatic coupling is realized by aligning a (single-mode) polymerwaveguide (PWG) array fabricated on a polymer with a silicon waveguide(SiWG) array fabricated on a silicon (Si) chip. The combined couplingportions in which this adiabatic coupling is provided are secured by anoptical epoxy or a UV adhesive.

Further included are an MTP connector secured to the polymer and aninterposer secured to the silicon chip; these are encapsulated. Forexample, an adhesive filled with silica beads is used.

Further included are a heat sink secured to the silicon chip and a coverplate (the outer shell of the package); the whole body is covered by thecover plate. The cover plate functions as the outer shell of thepackage.

In accordance with a connotational relationship of this packagestructure, a fabrication (assembly) method may be provided whichincludes a sequence of the following steps 1 to 10.

(1) Firstly, a silicon (Si) chip is prepared which has a siliconwaveguide (SiWG) array fabricated therein.

(2) A polymer is prepared which has a (single-mode) polymer waveguide(PWG) array fabricated therein.

(3) The silicon chip and the polymer are aligned in a manner having aspatial relationship by which optical coupling can be realized.

(4) The silicon chip and the polymer are secured to each other by anoptical epoxy or a UV adhesive (while the state of alignment ismaintained).

(5) An MTP connector is prepared which is secured to the polymer.

(6) An interposer is prepared which is secured to the silicon chip.

(7) These are encapsulated. An adhesive filled with silica beads may beused.

(8) A heat sink is prepared which is secured to the silicon chip.

(9) A cover plate (the outer shell of the package) is prepared.

(10) The whole body is covered by the cover plate to form a packagestructure.

FIG. 2 is a top view for explaining overlapping of the silicon waveguide(SiWG) array and the polymer waveguide (PWG) array, and also forexplaining evanescent wave involved in adiabatic coupling.

These arrays are, as illustrated in FIG. 2(a), overlapped over apredetermined distance in an optical axis direction, whereby evanescentlight is captured and transmitted.

In the verification of the predetermined distance in an optical axisdirection for the present invention, length L is approximately 3 mm(fabrication error being ±20%). However, those skilled in the art maycalculate a theoretically optimum length based on the conditions.

This type of optical coupling is known as an adiabatic coupling.

The width of core of the silicon waveguide (SiWG) array is, asillustrated in FIG. 2(b), smaller than that of the polymer waveguide(PWG) array.

In the verification for the present invention, a case is verified inwhich the width of core of the (single-mode) polymer waveguide (PWG)array fabricated on the polymer is approximately 5 μm (fabrication errorbeing ±20%) and the width of core of the silicon waveguide (SiWG) arrayfabricated on the silicon (Si) chip is approximately several hundred nmto 1 μm (fabrication error being ±30%).

Bilateral optical transmission exists between the silicon waveguide(SiWG) array and the polymer waveguide (PWG) array. However, since ascale difference in the width of core exists between the SiWG array andthe PWG array, there is a difference in the size of optical loss.

Evanescent wave transmitted from the side of SiWG to the side of PWG iseasy to capture on the side of PWG. However, evanescent wave transmittedfrom the side of PWG to the side of SiWG is relatively difficult tocapture on the side of SiWG.

The verification indicates that when the combination of the widths ofcore above described is used, it is appropriate to set gap g to 3 μm orso (fabrication error being ±20%). However, the appropriate size of gapg is affected by the wavelength of transmitted light and the mode fielddiameter (MFD). Those skilled in the art may determine the appropriatesize of gap g.

FIG. 3 is a top view for explaining the importance of alignment foroverlapping of the silicon waveguide (SiWG) array and the polymerwaveguide (PWG) array.

A slight amount δx of misalignment illustrated in FIG. 3(a) causes aloss of light transmission.

A slight amount δθ of inclination illustrated in FIG. 3(b) causes a lossof light transmission.

It can be seen that when misalignment of δx and inclination of δθ are,as illustrated in FIG. 3(c), combined, a greater amount of loss arises.

From the above description, it is understood that the alignment is veryimportant for overlapping of the silicon waveguide (SiWG) array and thepolymer waveguide (PWG) array.

FIG. 4 is an overall view illustrating a combination of a stubfabricated on the polymer and a groove fabricated on the silicon chip,the combination allows alignment of the (single-mode) polymer waveguide(PWG) array fabricated on the polymer and the silicon waveguide (SiWG)array fabricated on the silicon (Si) chip whereby the adiabatic couplingis realized according to the present invention.

In the present invention, the (single-mode) polymer waveguide (PWG)array fabricated on the polymer and the silicon waveguide (SiWG) arrayfabricated on the silicon (Si) chip are self-aligned according to thegroove and the stub highly precisely fabricated on both sides.

The sum of the present invention is that the groove and the stub aremade to function as the absolute positioning reference to perform theself-alignment according to the groove and the stub.

FIG. 5 is a view illustrating the inventive method of highly preciselyfabricating on the polymer a (single-mode) polymer waveguide (PWG) arrayand the stub.

The (single-mode) polymer waveguide (PWG) array and the stub may behighly precisely fabricated in an integrated manner, so that the stuband the groove function as the absolute positioning reference.

(1) Firstly, an (under) clad layer is coated on the polymer.

(2) A first mask is prepared along the alignment base line above thecoated core layer and (under) clad layer.

(3) Multi-channel cores (or a core array) are fabricated on the cladlayer by a photolithography process with a first mask. Here, therefraction index of the core layer material is larger than that of theclad layer material.

(4) Using a same type polymer material as that of the core array, a baselayer used to fabricate the stub is coated so that the core array iscovered.

(5) A second mask (having an exposure pattern different from the firstmask) is prepared along the same alignment base line used to prepare thefirst mask. Here, it can be seen that, since the same alignment baseline (for mask) is used, when the distance between the stub used as theabsolute positioning reference and the multi-channel cores (the corearray or one of the cores in the array) is x (FIG. 4), the positioningerror in x-axis δx is minimized so that the assembly is highly preciselyperformed.

(6) The stub is fabricated by a photolithography process with a secondmask.

Both the material of the core used to fabricate the core array by thephotolithography process with the first mask and the material of thebase layer used to fabricate the stub by the photolithography processwith the second mask may be selected from same type polymer materials,such as acrylic, epoxy, or polyimide.

Preferably, developer and rinse liquid used in the photolithographyprocess using the first mask is similar to those used in thephotolithography process using the second mask. This is needed to ensurethat the core array fabricated by the photolithography process with thefirst mask maintain their shape in the developing step of thephotolithography process with the second mask. When the developer andrinse liquid are used again as it is, the whole process becomes simpler.

FIG. 6 is a view for explaining respective advantages of PWG patterningby photolithography and PWG patterning by nano-imprint when the polymerwaveguide (PWG) array and the stub are fabricated according to thepresent invention.

In both the PWG patterning by photolithography and the PWG patterning bynano-imprint, a straight portion and a tapered portion may be providedin the stub.

The provision of the tapered portion in the stub is advantageous inthat, when the stub is inserted in a sliding manner into the groovefabricated on the silicon chip, the insertion is more easily performed.

In the PWG patterning by photolithography, the thickness control of thestub may be precisely performed by a spin coat process.

In the PWG patterning by nano-imprint, a metal cast is used which isprepared in advance by a precise cutting process; accordingly, thethickness control of the stub and the fabrication of a complicatedstructure of the stub in a height direction can be performed with asub-micro meter precision. Thus, a chamfer can be provided with highprecision.

The provision of the chamfer in the stub is advantageous in that, whenthe stub is placed or thrusted into the groove fabricated on the siliconchip, the placement or insertion is more easily performed.

In this way, when the PWG patterning by photolithography or nano-imprintis used, the insertion can be more easily performed in a sliding orthrusting manner. Furthermore, when the distance between the stub usedas the absolute positioning reference and the core array (one of thecores) is x in horizontal direction and y in vertical direction (FIG.4), the positioning error in x-axis δx and that in y-axis δy areminimized so that the assembly is highly precisely performed.

The groove fabricated on the silicon chip may be highly preciselyfabricated by etching or the like along with the silicon waveguide(SiWG) array fabricated on the silicon (Si) chip.

FIG. 7 is a view for explaining respective methods of PWG patterning byphotolithography and PWG patterning by nano-imprint when the polymerwaveguide (PWG) array and the stub are fabricated according to thepresent invention.

In the PWG patterning by nano-imprint, (1) Firstly, the base layer ofpolymer is prepared. Alternatively, a polymer may be coated on a glasssubstrate.

(2) A cast having a groove corresponding to the core of the(single-mode) polymer waveguide (PWG) array and a groove correspondingto the stub thereof is placed on the base layer of polymer. The cast maybe of metal. It is known that when the metal cast is used, a highprecision is easily achieved.

(3) The base layer of polymer is hardened. UV exposure may be performed.The polymer (the base layer) and the glass substrate may beUV-transparent.

(4) The cast is removed from the hardened base layer of polymer.

FIG. 8 is a view for explaining a method of aligning the siliconwaveguide (SiWG) array and the polymer waveguide (PWG) array and thensecuring the arrays, and for explaining the state after the arrays havebeen secured.

The package structure including the adiabatic coupling providedaccording to the present invention may also be provided as a fabricationmethod according to the connotational relationship described in FIG. 1.

What is claimed is:
 1. A method of fabricating on a polymer a(single-mode) polymer waveguide (PWG) array and a stub so that the(single-mode) polymer waveguide (PWG) array and the stub are alignedwith a silicon waveguide (SiWG) array fabricated on a silicon (Si) chipand a groove fabricated along a direction in which the SiWG isfabricated, whereby an adiabatic coupling is realized, the methodcomprising the steps of: preparing a polymer base layer; placing on thepolymer base layer a cast having a groove corresponding to a core of the(single-mode) polymer waveguide (PWG) array and a groove correspondingto the stub; hardening the polymer base layer; and removing the castfrom the hardened polymer base layer.
 2. The method according to claim1, further comprising the steps of: preparing the silicon (Si) chiphaving the silicon waveguide (SiWG) array and the groove fabricatedthereon; preparing the polymer having the (single-mode) polymerwaveguide (PWG) array and the stub fabricated thereon; aligning thesilicon chip with the polymer so that a spatial relationship is providedto realize the adiabatic coupling; and securing the silicon chip and thepolymer by use of an optical epoxy or a UV adhesive.
 3. The methodaccording to claim 2, further comprising the steps of: preparing an MTPconnector secured to the polymer; preparing an interposer secured to thesilicon chip; and encapsulating these.
 4. The method according to claim3, further comprising the steps of: preparing a heat sink secured to thesilicon chip; preparing a cover plate (the outer shell of a package);and covering the whole body by the cover plate.
 5. The method accordingto claim 1, wherein the width of core of the (single-mode) polymerwaveguide (PWG) array fabricated on the polymer is approximately 5 μm,and the width of core of the silicon waveguide (SiWG) array fabricatedon the silicon (Si) chip is approximately several hundred nm to 1 μm. 6.The method according to claim 1, wherein both the material of the coreused to fabricate the core array by the photolithography process withthe first mask and the material of the base layer used to fabricate thestub by the photolithography process with the second mask are selectedfrom the same type polymer materials, such as acrylic, epoxy orpolyimide, and wherein a developer and a rinse liquid used in thephotolithography process with the first mask can be used again as it isas a developer and a rinse liquid used in the photolithography processwith the second mask.
 7. A combination of a stub fabricated on a polymerand a groove fabricated on a silicon (Si) chip, prepared by a method offabricating on a polymer a (single-mode) polymer waveguide (PWG) arrayand a stub so that the (single-mode) polymer waveguide (PWG) array andthe stub are aligned with a silicon waveguide (SiWG) array fabricated ona silicon (Si) chip and a groove fabricated along a direction in whichthe SiWG is fabricated, whereby an adiabatic coupling is realized, themethod comprising the steps of: preparing a polymer base layer; placingon the polymer base layer a cast having a groove corresponding to a coreof the (single-mode) polymer waveguide (PWG) array and a groovecorresponding to the stub; hardening the polymer base layer; andremoving the cast from the hardened polymer base layer.
 8. The methodaccording to claim 7, further comprising the steps of: preparing thesilicon (Si) chip having the silicon waveguide (SiWG) array and thegroove fabricated thereon; preparing the polymer having the(single-mode) polymer waveguide (PWG) array and the stub fabricatedthereon; aligning the silicon chip with the polymer so that a spatialrelationship is provided to realize the adiabatic coupling; and securingthe silicon chip and the polymer by use of an optical epoxy or a UVadhesive.
 9. The method according to claim 7, further comprising thesteps of: preparing an MTP connector secured to the polymer; preparingan interposer secured to the silicon chip; and encapsulating these. 10.The method according to claim 7, further comprising the steps of:preparing a heat sink secured to the silicon chip; preparing a coverplate (the outer shell of a package); and covering the whole body by thecover plate.
 11. The method according to claim 7, wherein the width ofcore of the (single-mode) polymer waveguide (PWG) array fabricated onthe polymer is approximately 5 μm, and the width of core of the siliconwaveguide (SiWG) array fabricated on the silicon (Si) chip isapproximately several hundred nm to 1 μm.
 12. The method according toclaim 7, wherein both the material of the core used to fabricate thecore array by the photolithography process with the first mask and thematerial of the base layer used to fabricate the stub by thephotolithography process with the second mask are selected from the sametype polymer materials, such as acrylic, epoxy or polyimide, and whereina developer and a rinse liquid used in the photolithography process withthe first mask can be used again as it is as a developer and a rinseliquid used in the photolithography process with the second mask.